1. Field of the Invention
The present invention relates to a semiconductor light emitting element, and particularly to a semiconductor light emitting element capable of controlling spectral distribution.
The present invention also relates to a light source apparatus equipped with a semiconductor light emitting element.
The present invention further relates to an optical tomography imaging apparatus.
2. Description of the Related Art
Recently, demand has been growing for low cost multiple wavelength light sources in the fields of optical communications, measurement, and medicine, in which diagnosis and the like are performed using light. A light source for OCT (Optical Coherence Tomography), which is in use for wavelength inspection in optical communications, fiber gyros, OTDR measurement, and opthalmology, is a specific example of such a light source. It is expected that SLD's (Super Luminescent Diodes), which have a high probability of becoming capable of being produced at low cost, will serve as this type of multiple wavelength light source.
SLD's emit light that exhibits incoherent properties, which are similar to those of light emitted by regular light emitting diodes. SLD's emit light which has a broad bandwidth spectral distribution and are capable of light output of 1 mW or greater, similar to semiconductor lasers. Similar to semiconductor lasers, SLD's are equipped with a mechanism in which naturally discharged light, generated by recombination of injected carriers, is amplified while propagating toward a light emitting facet by high gain stimulated emission, then emitted from the light emitting facet.
There are known semiconductor light emitting elements, such as SLD's, having active layers structured to generate light having different gain wavelengths along directions in which waveguide paths extend. The structuring of the active layers in such a mariner is a method by which spectral distributions over broader bandwidths (across broader wavelength ranges) can be obtained. For example, Japanese Unexamined Patent Publication No. 6(1994)-196809 discloses a technique that utilizes selective growth to modulate the thickness of a quantum well active layer along a wave guiding direction of light. In this technique, two parallel stripe shaped SiO2 masks are formed on a layer surface with a constant interval therebetween. The film thickness and the composition of the active layer are changed in the axial direction of a cavity by changing the mask widths. Japanese Unexamined Patent Publication No. 6 (1994)-196809 suggests that the film thickness of only desired layers can be modulated, by alternately employing an atomic layer epitaxy mode and a standard mode, in the case that metal organic vapor phase epitaxy is employed.
An SLD produced employing the above technique, having a structure that generates light having different gain wavelengths along a wave guiding direction can obtain a broader bandwidth spectral distribution, compared to a standard SLD that generates light having a single gain wavelength. On the other hand, the spectral distribution of such an SLD is not a Gaussian distribution, and becomes that which has asymmetrical convexities and concavities (refer to curve B in the graph of FIG. 3).
It is important for light to be employed for measurement that utilizes light interference, such as OCT, to have a spectral distribution that approximates a Gaussian distribution. Therefore, in the case that an SLD produced by the aforementioned technique is employed as the light source for such a measuring apparatus, it becomes necessary to provide an optical filter for shaping the spectral distribution of emitted light into a Gaussian distribution.
Broad bandwidth spectral distributions are also desired in variable wavelength lasers equipped with semiconductor light emitting elements. Use of an SLD having a broad bandwidth gain wavelength structure produced by the aforementioned technique may be considered for this application. In addition to being able to emit light over a broad bandwidth, it is important for variable wavelength laser light sources to have uniform output across the broad bandwidth. Therefore, it becomes necessary to provide an optical filter that uniformizes light output across a variable output region.
In both light sources equipped with the aforementioned semiconductor light emitting element, it becomes necessary to further provide an optical filter for each semiconductor light emitting element, in order to shape the spectral distribution thereof to a desired spectral distribution. Therefore, the optical filters need to be designed and produced, thereby increasing the cost of the light sources.